# Photographic Myth Buster #24

#24. The Photosensitivity (Film Speed) and the Midtone Exposure

True or False?

The photosensitivity S, which is usually calculated from the speed point exposure Hsp through the equation S = Ho / Hsp, is independent of the midtone exposure.

False.

The midtone exposure Hm can be defined relative to the speed point exposure Hsp using a midtone shift M.

Hm = M Hsp

M = Hm / Hsp

Like the photosensitivity S, the midtone shift is a quantity without physical units that moves you from one exposure to another.

S = Ho / Hsp

Ho = S Hsp

In the case of the photosensitivity, you move from the the speed point exposure (back) to the reference exposure. In the case of the midtone shift, you move from the speed point exposure to the midtone exposure.

An interesting but seldom recognized feature of the standards for exposure meters is the requirement is that the reference exposure Ho and the midtone shift M of any (emulsion or solid-state) photosensitive array are related through a constant (actually two constants):

Ho M = qo K

Ho = qo K / M

The constant K is the exposure meter constant that appears in the exposure equation with which many photographers are familiar. The exposure meter constant with the conversion constant qo are together still a constant,

Substituting these two equations into the equation for the photosensitivity S

S = Ho / Hsp = (qo K / M) / Hsp = qo K / M Hsp = qo K / Hm

shows that the photosensitivity is not only dependent on the midtone exposure Hm but that the photosensitivity is completely determined by the midtone exposure.

Copyright 2008 Michael G. Prais, Ph.D.

For a readable but in-depth analysis of this concept along with many other concepts associated with photographic exposure, take a look at the book Photographic Exposure Calculations and Camera Operation. This book provides insight into the equations that govern exposure, exposure meters, photosensitive arrays (both solid-state and emulsion) and the Zone System as well as concepts associated with resolution, dynamic range, and depth of field.

The book is available through Amazon.com (ISBN 978-1-4392-0641-6) where you can Search Inside!™.

Check http://michaelprais.info under Photography for the table of contents, an extensive list of the topics and subtopics covered, the preface describing the purpose of the book, and a diagram central to the concepts in the book.